The transmission capacity of optical transmission systems can be increased by multiplexing channels, which have a number of different wavelengths, of an optical wavelength division multiplex (WDM) signal. Due to the wavelength dependency of optical components and effects, the channels have different levels, signal-to-noise ratios and/or signal distortion at the end of the transmission path; for example, in the form of nonlinearities. Ideally, the channels should have a flat gain spectrum after being amplified one or more times along the transmission path. By way of example, a transmission system having a number of cascaded Raman amplifiers has been proposed in Patent Application DE 10057659.1, in order to achieve the flat gain spectrum.
In conventional WDM transmission systems, identical channel levels are set for all the channels at the start of the transmission path via variable attenuaters. However, the wavelength dependency of the optical components results in channel level differences increasing virtually continuously along the path.
Better utilization of the system resources is achieved by the use of pre-emphasis for the channel levels at the start of the transmission path. The distribution of the channel levels is chosen at the start of the path such that all the channels have the same signal-to-noise ratios OSNR at the end of the path. This OSNR pre-emphasis with signal-to-noise ratio control is highly suitable when there is little signal distortion or for so-called noise-limited systems.
A further improved method is the Q-pre-emphasis proposed in Patent Application DE 10047342.3, in which the level distribution of the channels at the start of the transmission path is chosen such that the channels have the same values of the Q factor at the end of the transmission path (see “Optical Fiber Telecommunications”, IIIA, I. P. Kaminow, T. L. Kich, p. 316, 1997, ISBN 0-12-395170-4). The channels with a poor Q factor are raised at the start of the transmission path in order to compensate for the poor signal quality by improved signal-to-noise ratios OSNR at the path end. Since the Q-factor represents a direct measure of the signal quality, the Q-pre-emphasis has the advantage over the previously cited OSNR pre-emphasis that, in addition to the OSNR compensation, a large number of additional limiting effects are taken into account, such as nonlinear signal distortion, different dispersion compensation between the channels, different transmitter or receiver characteristics, and crosstalk due to multiple reflections.
In addition to the Q-factor as a measure of the signal quality, equivalent quality parameters also can be used, such as the bit error rate BER or the number of corrected bits in systems using forward error correction (FEC).
In systems with high channel levels at the start of the transmission path, OSNR-pre-emphasis or Q-pre-emphasis results in high levels of signal distortion due to nonlinear effects such as Four Wave Mixing, Self Phase Modulation, Cross Phase Modulation, Stimulated Raman Scattering (see “Fiber-Optic Communication Systems”, G. P. Agrawal, 2nd Edition, pp. 323-328). This nonlinear distortion is not compensated for by improving the signal-to-noise ratios OSNR when increasing individual channel levels. FIG. 1 shows the Q-factor for optimal dispersion in a transmission system having 8 channels (100 GHz channel separation, 10 Gb/s data rate and NRZ coding), as a function of the input power Pin of the channels. The channel quality Q initially rises linearly as the input power Pin increases, but enters saturation with the nonlinear effect becoming increasingly significant at high input power levels and then falls again for even higher power levels.
An object of the present invention is, therefore, to ensure an improvement in the transmission quality for a wavelength division multiplex system with broadband cascaded Raman amplifiers; in particular, in the presence of high levels of nonlinear signal distortion.